In a microstructure such as a template for imprinting, a mask substrate for photolithography, or a semiconductor wafer, a minute concave convex portion is formed on a surface of a substrate.
Here, as a method of removing a contaminant such as a particle adhered to the surface of the substrate, an ultrasonic cleaning method, a two-fluid spray cleaning method, etc. are known. However, if an ultrasonic wave is applied to the substrate or a fluid is sprayed onto the surface of the substrate, the minute concave convex portion formed on the surface of the substrate may be damaged. Further, recently, miniaturization of the concave convex portion has progressed, and the concave convex portion is more likely to be damaged.
Therefore, as a method of removing a contaminant adhered to the surface of the substrate, a freeze cleaning method has been proposed.
In the freeze cleaning method, first, pure water is supplied to the surface of the rotated substrate, and a part of the supplied pure water is discharged to form a water film on the surface of the substrate. Subsequently, a cooling gas is supplied to the substrate on a side where the water film is formed to freeze the water film. When the water film is frozen and an ice film is formed, the contaminant is incorporated into the ice film, and therefore, the contaminant is separated from the surface of the substrate. Subsequently, pure water is supplied to the ice film to thaw the ice film, and the contaminant is removed from the surface of the substrate along with the pure water.
According to the freeze cleaning method, the minute concave convex portion formed on the surface of the substrate can be prevented from being damaged.
However, when the cooling gas is supplied to the substrate on a side where the water film is formed, freezing starts from a side of the surface of the water film (on an opposite side to the substrate of the water film). When freezing starts from the side of the surface of the water film, it becomes difficult to separate an impurity adhered to the surface of the substrate from the surface of the substrate. Therefore, it was difficult to improve a contaminant removal efficiency.